1. Field of the Invention
The present invention relates to a vehicle passenger detection device and particularly to a vehicle passenger detection device that detects the presence of a passenger upon a seat.
2. Background
Known examples of such a vehicle passenger detection device include, for example, the device taught by Japanese Unexamined Patent Publication No. 2000-258233. With such a vehicle passenger detection device, a strain gage-type weight measurement device 3 is provided between a seat frame 1 (or the lower rail of the seat slide rails) provided in the lower part of the seat and a seat bracket 2 attached to the floor panel.
This weight measurement device 3 consists of a base member 4 provided with a pin hole 4a and slot 4b, an arm 7 pivotally supported upon the base member 4 using a base pin 5 serving as the fulcrum (support axle) and a retainer 6, a flat spring 10 that is attached to one end of the arm 7 with a bracket pin 8 and retainer 9, a pin bracket 11 as a load transfer member and a strain gage-type load sensor 12. In FIG. 1, F indicates the front of the vehicle while R indicates the rear of the vehicle.
With this device, when the load of the passenger upon the seat is transferred via the pin bracket 11, flat spring 10 and bracket pin 8 to the end of the arm 7, this arm 7 is pivotally supported by the base pin 5, so it rocks about the base pin 5, the motion of the other end of the arm 7 is transferred to the load sensor 12 and the load of the passenger is detected by the load sensor 12. With this device, in order to prevent excessive rocking of the arm 7, bushings are disposed between the arm 7 and base member 4, and moreover, the base pin 5 on the fulcrum side is pivotally supported in the pin hole 4a so that backlash does not occur.
However, with a weight measurement device such as that shown in FIG. 1, mechanical hysteresis occurs.
Mechanical hysteresis refers to a situation wherein, when the passenger exits the vehicle, the load on the seat is released and the seat returns to its original position, and at the stage when the load on the weight measurement device reaches a very small load of roughly 2 kg, for example, this very small load balances against the mechanical resistance of the moving parts within the weight measurement device, and thus the weight measurement device does not return to the zero position as its original position. As a result, even though the passenger is no longer present upon the seat, a constant load continues to be input to the load measurement device of the vehicle passenger detection device.
The results of detection by the vehicle passenger detection device are used to control airbag deployment, so the vehicle passenger detection device is required in order to determine the presence of a passenger upon the seat and whether the passenger is an adult or child. With a vehicle passenger detection device as described above, a determination is made among the states of no passenger present or an adult or child passenger present based on the load detected by the weight measurement device and predetermined threshold values.
Children are light in weight, so the threshold value for distinguishing between the states of no passenger present and a child passenger present is set to a relatively small value. For this reason, in the state in which mechanical hysteresis causes a constant load to be input to the load measurement device of the vehicle passenger detection device, if cargo is placed upon the seat, then the total load of the load due to the mechanical hysteresis and the load due to cargo may exceed the threshold value for distinguishing the state of no passenger present from the state in which a child passenger is present in the load measurement device of the vehicle passenger detection device. As a result, there is a problem in that the vehicle passenger detection device may erroneously determine that a child is sitting on that seat even though no passenger is present.